This invention generally relates to moisture removal from vessels and more particularly relates to a system and method for evaporating moisture from a gap defined between a first tubular member, such as a heat transfer tube, surrounding a second tubular member, such as a repair sleeve, in a nuclear steam generator.
Occasionally, nuclear steam generator heat transfer tubes may degrade and thus may not remain leak-tight. If through-wall cracking occurs due to the degradation, the radioactive primary fluid flowing through the tubes may leak through the crack and commingle with the nonradioactive secondary fluid surrounding the tubes, a highly undesirable result.
However, the degraded tube may remain in service by sleeving the degraded portion of the tube. When sleeving is performed, a tubular repair sleeve is inserted into the heat transfer tube to span or cover the degraded portion of the tube. The sleeve is then radially expanded into intimate engagement with the wall of the tube to secure the sleeve to the tube. The radial expansion of the sleeve may be accomplished by means of an hydraulic expansion mandrel, such that a sleeve-to-tube hydraulic expansion joint is defined where the sleeve has been expanded into engagement with the tube.
However, after hydraulic expansion, a small annular gap (e.g., approximately 0.012 inch) may nonetheless exist between the sleeve and the wall of the tube and may extend from the top end of the sleeve to a few inches (e.g., approximately six inches) below the top end of the sleeve. Thus, due to the presence of the annular gap, the sleeve will not engage the tube from the top end of the sleeve to that point which is a few inches below the top end of the sleeve. In other words, the sleeve will intimately engage the tube only starting at a transition region thereof beginning a few inches below the top end of the sleeve (termed herein "the hydraulic expansion transition region").
However, it has been observed that during subsequent operation of the steam generator, additional cracking may initiate in the surface of the heat transfer tube near the region of the hydraulic expansion transition region. Therefore, a portion of the hydraulic expansion transition region is further hydraulically expanded into engagement with the inside surface of the tube to cover such additional cracking.
After the previously mentioned expansion of the hydraulic expansion transition region is accomplished, a suitable welding device, such as a laser welding device, may then be inserted into the sleeve to weld the sleeve to the tube at the hydraulic expansion transition region. In this manner, the welding device sealingly affixes the sleeve to the tube.
It is important to sealingly affix the sleeve to the tube in order to prevent commingling the nonradioactive secondary fluid with the radioactive primary fluid. However, the gap and the cracks in the inside surface of the tube will typically contain moisture in the form of liquid water or a steam/water mixture. It is known that the presence of significant amounts of moisture in the gap and cracks will interfere with satisfactorily welding the sleeve to the tube.
Therefore, it is desirable to evaporate (i.e., boil) or otherwise remove the moisture from the gap (including tube wall cracks), before the hydraulic expansion transition region is expanded into engagement with the tube wall which occurs prior to welding the sleeve to the tube. This is important in order to obtain a satisfactory weldment after expansion. If a satisfactory weldment is not obtained, the weldment may have to be ground-out and reapplied. On occasion, the weldment may have to be repeatedly reapplied due to persistent moisture in the gap (including cracks). This is undesirable because repeated rewelding of the hydraulic expansion joint may cause unacceptable delay in returning the steam generator to service. Delay in returning the steam generator to service is not cost-effective because each day of delay in returning the steam generator to service costs the reactor owner approximately $500,000 per day in replacement power costs. Consequently, it is desirable to remove or evaporate substantially all the moisture from the gap in order to form a quality weldment the first time a weldment is attempted.
Therefore, what is needed are a system and method for evaporating moisture from a gap defined between a first tubular member, such as a heat transfer tube, surrounding a second tubular member, such as a repair sleeve, in a nuclear steam generator.